Stem cells provide the sole source of new cells in adult organisms, producing both self-renewing stem cells and daughter cells that differentiate. Drosophila and mouse spermatogenesis are model stem cell systems with different and complementary strengths; therefore comparison between these two systems will be used to identify conserved mechanisms that control stem cell behavior. Using gene expression prolife lists, 6 signaling pathways have been identified that are predicted to be enriched in germline stem cells or the somatic niche in both species. In Aim 1, the expression of pathway members will be verified by testes in situ hybridization in both species. In Aim 2, pathways with expression in mouse and fly will be tested for function in spermatogenic stem cells. This will involve loss- and gain-of-function techniques in Drosophiia including the use of RNAi, transgenics, and mutant clones, and a stem cell transplantation assay to assess function of Lentivirus-modified cells in mouse. Better understanding of the molecular mechanisms controlling these stem cells will aid in the understanding and treatment of infertility, testicular cancer, and genetically inherited disease, as well as contribute to a better understanding of the broader nature of stem cells.